ETBC: COLLABORATIVE RESEARCH: MASS-DEPENDENT AND INDEPENDENT MERCURY ISOTOPE FRACTIONATION DURING MICROBIAL METHYLATION AND REDOX TRANSFORMATIONS OF MERCURY IN NATURAL WATERS

智力价值：拟议的研究是基于PI和其他人的最新成果，并寻求扩大PI？微生物和非生物汞转化过程中的同位素分馏研究。在过去的五年中，PI表明：(I)具有汞还原酶的几株细菌对离子汞(Hg[II])的微生物还原过程中发生了依赖于质量的同位素分馏(MDF)，微生物对甲基汞(MeHg)的降解也发生了依赖于质量的同位素分馏(MDF)；(Ii)汞(II)的光还原和甲基汞的光降解导致了MDF以及高达2？在淡水和海水鱼类组织中记录的不同数量的MIF(分别表示为±199汞和±201汞)表明，在甲基汞进入水生食物网之前，它的光降解程度不同。这些结果以及Blum实验室和其他人记录的自然样品中的巨大变化强烈表明，汞的同位素组成有可能区分不同的汞(0)排放来源以及汞(II)还原和甲基汞降解的途径。研究的重点是汞的氧化还原转化和甲基汞的降解。然而，甲基汞在水生食物网中的生物积累对人类和生态系统的健康有着深远的影响，因此，检查汞甲基化过程中是否发生同位素分馏是一个高度优先的问题。如果甲基汞在形成过程中发生显著的分馏，并受到不同环境条件和甲基化过程性质的调节，则可利用工具来区分环境中甲基汞的来源和途径，并加强对汞污染生态系统的管理。这项研究的第一个目标是检查硫酸盐和铁还原细菌在汞甲基化过程中的同位素分馏，以验证微生物汞甲基化导致显著的MDF，而不是MIF的假设。第二个目标是研究定义淡水和海洋环境的环境变量在汞氧化还原转化过程中如何影响MIF和MDF，以验证光化学还原、氧化和去甲基化将在反应底物和产物上印记诊断MDF和MIF信号的假设。最后，由水生生态系统的重要组成部分--光养浮游生物所介导的转化途径中的汞同位素分馏迄今尚未被研究过。第三个目标解决了这种知识的缺乏，测试了这样的假设，即在光照下孵育的浮游植物细胞内汞(II)减少和甲基汞降解将导致MDF和可能的MIF。更广泛的影响：拟议的研究活动将继续为新的方法或确定受影响生态系统中汞转化的源、汇和路径奠定基础。这一方法有可能在从分子机制到生态系统、到全球循环和地质记录的时间和空间尺度上显著加强对汞生物地球化学的理解。由于生态系统汞污染仍然是一个主要的公共卫生问题，该项目将支持实施合理的环境实践，以减少汞污染和暴露。拟议的研究将培训一名博士后，了解稳定同位素方法在地球生物学和生态系统过程中的应用。此外，本科生和研究生将被整合到该项目中，让他们接触到生物学、地质学和生态系统科学之间的尖端概念和技术。正是在这个界面上，重要的范式转换和研究进展正在取得。作为高级论文研究项目和博士论文项目的一部分，本科生将协助学习分析地球化学。结果将被广泛出版和传播。

Intellectual merit: The proposed research is based on recent achievements by the PIs and others, and seeks to expand the PIs? on-going studies of isotopic fractionation during microbial and abiotic Hg transformations. During the last five years the PIs have shown that: (i) Mass dependent isotopic fractionation (MDF) occurred during the microbial reduction of ionic mercury (Hg[II]) by several bacterial strains that possess the enzyme mercuric reductase and MDF also occurred during microbial methylmercury (MeHg) degradation; (ii) Photoreduction of Hg(II) and photodegradation of MeHg caused MDF as well as mass independent fractionation (MIF) of up to 2?. Varying amounts of MIF (denoted as Ä199Hg and Ä201Hg) recorded in freshwater and marine fish tissue suggested different extents of photo degradation of MeHg prior to its incorporation into the aquatic food web. These results along with large variations in natural samples documented by the Blum lab and others, strongly suggest that the isotopic composition of Hg has the potential for distinguishing between different sources of Hg(0) emissions and pathways of Hg(II) reduction and MeHg degradation.To date, the PIs? studies have focused on Hg redox transformations and MeHg degradation. Yet, the bioaccumulation of MeHg in aquatic food webs has a profound effect on human and ecosystem health and the examination of whether or not isotope fractionation occurs during methylation of Hg is, therefore, a high priority. If significant fractionation occurs during formation of MeHg and is modulated by different environmental conditions and by the nature of the methylating processes, tools for distinguishing sources and pathways of MeHg in the environment may become available and enhance the management of Hg contaminated ecosystems. The first objective of the proposed study is the examination of isotopic fractionation during Hg methylation by sulfate and iron reducing bacteria to test the hypothesis that microbial Hg methylation results in significant MDF, but not in MIF. The second objective is the investigation ofhow environmental variables, which define freshwater and marine environments, impact MIF and MDF during Hg redox transformations to test the hypothesis that photochemical reduction, oxidation and demethylation will imprint diagnostic MDF and MIF signatures on reaction substrates and products. Finally, Hg isotopic fractionation during transformation pathways mediated by an important component of aquatic ecosystems, phototrophic planktonic organisms, has not been examined to date. The third objective addresses this lack of knowledge by testing the hypothesis that intracellular Hg(II) reduction and MeHg degradation in phytoplankton incubated in the light will result in MDF and possibly MIF. Broader Impact: The proposed research activity will continue to lay the groundwork for a new approach or the identification of sources, sinks, and pathways of Hg transformations in impacted ecosystems. This approach has the potential to significantly enhance understanding of Hg biogeochemistry on temporal and spatial scales ranging from molecular mechanisms, to ecosystems, to global cycles, and to the geological record. As ecosystem Hg contamination remains a major public health concern, this project will support implementation of sound environmental practices to reduce Hg contamination and exposure. The proposed research will train a postdoctoral fellow in the application of stable isotope-based approaches in geobiology and ecosystem processes. In addition, undergraduate and graduate students will be integrated into the project, exposing them to cutting edge concepts and technologies, which are at the interface between biology, geology and ecosystem sciences. It is at this interface that importantparadigm-shifting, research advances are being made. Undergraduate students will assist with the analytical geochemistry as part of senior thesis research projects and PhD dissertation projects. Results will be published and disseminated broadly.